As electronic devices become smaller in size, there is also a demand for batteries that are smaller in size and lighter in weight to serve as power sources for the electronic devices. Moreover, there is also a demand for batteries having higher energy densities and higher energy efficiencies. Lithium ion batteries and other nonaqueous electrolyte batteries are increasingly expected to meet such demands. Japanese Patent No. 3505905 discloses a nonaqueous electrolyte battery in which a positive electrode, a negative electrode, and an electrolytic solution are encapsulated in, for example, a bag-like package made of a laminated film containing a metal foil, and leads that are connected to the electrodes extend to the outside of the package.
Each of the leads connected to the positive or negative electrode has an insulator covering thereof at a portion where it is attached to the package to prevent electrical short circuiting between the leads and the metal foil of the laminated film. Also, at the portions, the leads are fused to prescribed positions of a border part of the package that is formed by fusing the laminated films, such that the leads extend to the outside of the package while maintaining the hermetically sealed state of the package.
The insulator used to cover each of the leads is made up of two layers: an inner insulation layer and an outer insulation layer that has a higher melting point than the inner insulation layer and does not melt at the temperature at which the laminated films are heat-sealed. The insulator is bonded to each of the metal leads at a portion where each of the leads is attached to the package, by heating and melting the inner insulation layer. The leads having insulators are arranged in lead passage portions of the package, such that the insulators are positioned at seal portions of the edge of the laminated film. Then, the edge of the laminated film is sealed by heat sealing. The outer insulation layers of the insulators do not melt at the temperature at which the heat sealing proceeds. Thus, the metal leads do not become exposed through the insulators during the heat sealing. As a result, electrical short circuiting does not occur between the metal leads and the metal foil inside the laminated film. The metal lead is made of aluminum, nickel (including nickel plating), or copper.
If the hermetic adhesion between the seal portion of the laminated film and the lead at a portion where the lead is attached to the package is insufficient, however, moisture will penetrate the inside of the battery from the outside and reacts with the electrolytic acid inside the battery, thereby producing hydrofluoric acid. In such cases, even though the surface of the metal lead is hermetically sealed by the insulator at the lead passage portion of the package, the surface of the metal lead will gradually corrode over a long period of time due to the hydrofluoric acid, and the portion of the metal lead to which the insulator is bonded will collapse. As a result, the airtightness of the package will be degraded and the battery characteristics will decline.
One method of solving such problem is to cover with a chemical conversion layer a surface of a metal lead (tab) at a portion to be sealed with the insulator, as shown in Japanese Patent Application Publication No. 2002-216741. Japanese Patent Application Publication No. 2002-216741 discloses a typical chromate phosphate treatment in which a coating is chemically created using an aqueous chromic acid solution containing a phosphate salt. It also discloses forming a chemical conversion coating on a lead by immersing the lead in a chemical conversion solution that includes a resin containing a phenol resin and a metal salt such as titanium, zircon, or other metal. In this manner, chromium-based chemical conversion treatments are effective as a chemical conversion treatment for imparting resistance against hydrofluoric acid, and have been used in many different fields. However, due to environmental pollution concerns, chromium-free chemical conversion treatments are being developed in various fields. It is anticipated that there will be a demand to develop chemical conversion treatments that are completely free of chromium in the future.
In fields other than the field of leads for nonaqueous electrolyte batteries, there have already been many attempts for practical use of salts containing metals such as zirconia-based and titanium-based metals instead of chromium as materials for making coatings. There have also been attempts to use a mixture of such salt with polyacrylic acid as a material for making coatings. Such coatings are reported to have satisfactory adhesion characteristics when applied to aluminum and also to be effective from the standpoint of corrosion resistance (based on an article on homepage of JPO website dated 28 Mar. 2003).
Patent Document 1: Japanese Patent No. 3505905
Patent Document 2: Japanese Patent Application Publication No. 2002-216741